This invention relates to a nitrogen oxide reducing catalyst suitable for reduction to N.sub.2 and O.sub.2 of a nitrogen oxide (NO.sub.x) in a gas exhausted from an internal combustion engine such as an automobile engine etc.
A gas exhausted from an internal combustion engine such as an automobile engine and gas turbine contains carbon monoxide (CO), nitrogen oxide (NO.sub.x) and a hydrocarbon which is produced from an incomplete combustion. Of these, the nitrogen oxide per se is noxious and is provides a cause for air pollution, since the nitrogen oxide produces, for example, a harmful smog through a photochemical reaction after emission into the atmosphere. A solution to this problem has been attempted by, for example, a method for controlling an amount of NO.sub.x through fuel control and a method for converting the nitrogen oxide to innoxious components, i.e., N.sub.2 and O.sub.2, using a catalyst. The catalyst method has received a wide attention as an effective solution, since it is simpler from the practical view point than the fuel control method such as an air-fuel ratio control method, water spray method etc.
However, this catalyst method does not provide a satisfactory solution to the above-mentioned problem. A noble metal series catalyst, in particular, such as Pd, Pt, Au etc. has a strong tendency to produce a large amount of by-product ammonia (NH.sub.3) in the reduction of NO.sub.x and the NH.sub.3 is converted upon contact with an oxide catalyst to a nitrogen oxide, presenting a secondary air contamination. A catalyst of a metal oxide such as an alumina ceramic, ferrite etc. exhibits a relatively high stability at high temperature, but it suffers a performance deterioration due to a water or a water vapor (this is usually contaned in an amount of 10 to 15% in an exhaust gas) included in an exhaust gas. For this reason, the catalyst can not effect a satisfactory reduction of NO.sub.x and nor can it be used for a longer period of time.
In reducing nitrogen oxide in the exhaust gas from the internal combustion to innoxious components by the catalyst method, the catalyst must have the following requirements:
A. it has an ability of completely reducing NO.sub.x to N.sub.2 and O.sub.2 PA1 b. it prevents production of ammonia even in coexistence with water, water vapor and hydrogen PA1 C. it exhibits high stability even at an exhaust gas temperature (usually 600.degree. to 1000.degree. C) and excellent resistance to oxidation and to corrosion.
Any known catalyst does not meet the above-mentioned requirements and for this reason the catalyst method has been used in combination with the fuel control method in reducing the NO.sub.x in the exhaust gas to innoxious elements.